Method of melting metals by use of an electric furnace



Sept. 27, 1938. J. A. ZUBLIN ET AL 2,131,396

METHOD OF MELTING METALS BY USE OF AN ELECTRIC FURNACE Filed March 30,1937 w L mm,

Patented Sept. 27, 1938 UNITED STATES to said Zublin 5 Claims.

The present invention relates to methods of melting metals by use ofelectric furnaces, especially those metals having extremely high meltingtemperatures, such as tungsten. A specific 5 application of our newmethod resides in the manufacture of tungsten carbide compounds and,

although the invention will be described with that aspect in mind, it isto be understood that it is in no manner limited thereto.

In order to accomplish the melt successfully in the production of acommercial tungsten carbide alloy (referred to herein as simply tungstencarbide, since that is the predominant ingredient) it is necessary tocarry out the final stages 6 of heating very rapidly, which requires theuse of equipment'capable of a very high rate of energy'or heat input tothe furnace and charge.

The rapidity with which tungsten absorbs carbon, and the large extent ofradiation of heat I from the crucible occasions the use of this highrate of heat input to reach the required melting temperature. It isfurther essential that this since excess carbon makes the alloy toobrittle.

An opposite condition is required during the preliminary stages ofmelting. It the heat were applied initially at the same rate as bestsuited for final heating, the material would be heated 7 too rapidly,and much or all of the powdered 0 charge would be blown out the ventholes of the crucible by the gases evolved by heating. By heating thecharge at a lower rate, blowing can be prevented, because the gaseswould be generated over a longer period of time so that their escapewould be less rapid and violent. In addition, blowing is materiallyreduced apparently by a preliminary sinterlng or alloying oi theelements having lower melting points that binds the charge together intoa more or less unitary mass. It is further believed that heating at aslow initial rate lowers the final melting point of the charge, thisphenomenon resulting apparently from either an alloying of the solidtungsten, having a melting point oi 3370 C., with the molten elements;or a pre-carburizing of the tungsten that forms a carbide of lowermelting point than the tungsten alone, being about 2700 C.

The rates of initial and final heatings must be made variable, beingdependent upon several factors, including the weight of the charge, theelements contained therein, the proportions used,

and the character of theresulting product.

To satisfy the above noted conflicting conditions of low initial heatingrate, higher final a heating rate. and variability in those rates, ithas heat input be applied within exact time limits,

PATENT OFFICE METHOD OF MELTING METALS BY USE OF AN ELECTRIC FURNACEJohn A. Zublin, Los Angeles, and John S. Goodwin, Alhambra, Callt; saidGoodwin assignor Application March 30, 1937, Serial No. 133,872

been necessary heretofore to provide expensive and complicated equipmentcapable of furnishing a current varying as the heat input requirementschange from a low initial rate to a much higher final rate. Thesecomplications of equipment are normally attended by large power lossesand poor electrical efiiciency, conditions which are obviouslyundesirable. Additionally, if it is desired to permit flexible operationcovering wide ranges in the rates of heating, the undesirable conditionsare amplified by the necessity for additional expensive equipment.

Accordingly, it is an object of our invention to be able to utilizesimple equipment in the process of heating the charge.

It is a further object of our invention to devise a method of usingsimple heating equipment in such manner as to heat the charge atdifierent rates.

It is a further object of our invention to devise a method of usingsimple heating equipment having a. constant rate of heat input in suchmanner as to heat the charge at difierent rates.

It is a further object of our invention to devise a method of usingsimple heating equipment in such manner as to provide flexibility in theheating rates of the charge.

It is a further object of our invention to devise a method of operatingan electric furnace in such manner as to provide variability at will inthe heating rates of the charge.

It is a further object of our invention to form a tungsten carbide alloyby the use of simple heating equipment.

The manner of attaining the above and other objects of our invention canbe understood from a consideration of the following description andannexed drawing, wherein:

The single figure is a side view of an electric furnace with thecrucible shown between the electrodes.

The device illustrated on the drawing includes a supporting structure Ifrom which extends an upright 2. Mounted on the supporting structure isa lower electrode 3 and pivotally supported by the upright is an uppermovable electrode 4. This latter electrode is fixed to a plate 5parallel to the upright and pivoted at the point 6 to a manuallyoperable lever 1. The lever I is pivoted to the upright by a pin 8, andthe plate 5 is connected to the upright by means of a link 9 connectedto the upright by the pin l0 and to the plate by the pin H. The abovedescribed lever and linkage arrangement constitutes a parallel motiondevice, with which the uppe electrode contact with the carbon cruciblel2 placed on the' lower electrode 3. The crucible includes a. containerfor the charge 12a, a lid [2b engageable with the upper electrode, andone or more vents 120 through which gases may escape during heating.

Current is supplied to the electric furnace from a suitable sourcethrough the control switch 13 to the step down transformer I4, having aprimary winding l5 and a secondary winding l6. From the secondarywinding of the transformer flexible leads I1, I! are connected to clampsI8, l9 fitted around the electrodes 3, 4 respectively.

With the main switch l3 closed, and with the apparatus in a positiondisclosed on the drawing, current will fiow through the carbon cruciblel2 and heat will be applied to the crucible and its contents. Uponmoving the lever I upwardly, contact between the .upper electrode 4 andthe crucible l2 can be broken, and, consequently, the heat imparted tothe crucible will be discontinued.

By intermittently manipulating the lever' l so as to make and breakcontact between the upper electrode and the crucible l2, current will beapplied intermittently to the crucible, and the crucible will have heatimparted to it at intervals. If contact between the upper electrode andthe crucible were maintained continuously for a fixed period of time,for a substantially constant current value a certain amount of heatwould be imparted to the crucible and its contents. During an equalperiod of time, and with the same current value, if contact were to bebroken at intervals, the time during which current would flow throughthe crucible would be less than the above named period. Therefore, lessheat would be imparted to the crucible, and its temperature would not beraised to the same degree as when current was applied continually duringthe period.

Accordingly, thev rate of temperature rise of crucible l2 and itscontents can be controlled with an approximately constant currentsupplied by the secondary winding of the transformer. All that isrequired is that the electrode make contact with the crucible for arelatively short period of time, and then such contact be broken for aperiod of time. Subsequently, contact will again be made,'and this cycleof operation would be repeated until the desired temperature in thecrucible is obtainable and the desired rate of temperature riseeffected.

While the temperature at the outside of the crucible fluctuates becauseof radiation losses during the nonheating intervals, the temperature atthe center of the charge increases at a substantially uniform rate,because the crucible walls become much hotter than the charge at the endof each heating period and heat is transmitted inwardly to the chargeduring the intervals between heating periods.

By intermittently making and breaking the circuit through the crucible,it is possible to use the simple electric furnace disclosed to controlthe rate of temperature rise and the final temperature reached. If it isdesired to heat the crucible rapidly, contact between the upperelectrode and the crucible would be maintained for longer periods andthe breaking of the contact would occur for shorter periods. Theconverse would be true if the rate of heating of the crucible weredesired to be less. It will be seen, therefore, that the manner ofoperating the electric furnace possesses flexibility to a high degree.

The apparatus and method of operation can be used in a variety ofprocesses. For the purpose, of illustration, we shall describe in detaila particular process of manufacturing tungsten carbide, which is typicalof our improved process, though the process is no way limited to themanufacture of that particular product.

The charge of powdered metals includes primarily tungsten and smallproportions of one or more of such elements as copper, molybdenum, ornickel, as well as a very minor amount, about 1% to 2% usually, ofimpurities inherent in commercial grades of those metals. The charge isput in the carbon crucible l2, which is placed between the twoelectrodes 3, 4.

As the heat is applied, the powdered charge is agitated by the evolutionof gas as the impurities vaporize. The initial heating must be at a ratesufficiently low to prevent the gases from blowing the charge out thevents, since it is possible to blow an entirecharge from the crucible byapplying heat at an excessively high rate. By using the above describedsimplified apparatus, current is supplied to the electrodes at only oneapproximately constant rate, the rate of heating being controlled andreduced to any desired low value by applying the current intermittentlyfor predetermined periods.

By way of example, a charge of about three pounds is placed in thecrucible and a current of about 18,000 to 20,000 amperes at 20 volts isapplied at the beginning of the preliminary heat period. The actualaverage rate of heating is but a small part of the maximum ratepossible, since for a period of two minutes the current is applied foronly one second out of each fifteen seconds. In between these one secondperiods of heating, the circuit is broken so that no heating takes placeduring the intervals. The circuit could be broken in any suitable manneror place, but for convenience we prefer to break it at the crucible bybreaking the contact of one electrode with the crucible. After the firstperiod of two minutes, the rate of heating is increased to one second ineach ten seconds for a period of two minutes, after which the rate isagain increased to one second in each seven and one-half seconds andmaintained at this rate for one minute, followed by a further increaseto one second in each five seconds for a period of one minute. At theend of the six minutes of pre liminary heating a temperature of around2,000"

C. has been reached in the charge, and the impurities nearly all drivenofl as gases, so that blowing has nearly stopped. However, the

' vaporization point of the metals to be retained in the final alloy hasnot been reached, although practically all of the charge except thetungsten has become molten.

At the end of the preliminary heating period,

' the circuit is held closed continuously so that in any furnace, novariation in current is made in our process, except that necessarilyaccompanying the resistance change of the crucible and charge, which iscaused by their temperature change. In view of this circumstance, thecurrent used may be described as being of generally constant value.

By our method of operation, a very simple electric furnace can be usedhaving. no means of direct control of the current, and in which a chargeof materials having different melting points can be fused, regardless ofwhether their melting points are high or low.. The charge can be heatedat a suitably slow initial speed with a current which is sufficient forthe very rapid rise to extremely high temperatures necessary in makingalloys of tungsten carbide. The average rate of heating is reduced bylimiting the length of the heating periods, and any rate desired can beeasily and immediately obtained by varying the time of the heatingperiods, or the length of the intervals between heating periods, orboth. Thus, without expensive and complicated equipment, the method ofoperating the furnace is made flexible, adaptable to any needs, andfully under the control of the operator at all times.

Various changes may be made and still fall within the scope of ourinvention. Thus, the preliminary heating may involve any other numberand combination of heating rates than the four described. Also, thecurrent may be broken at any place, either in the primary or secondarycircuits of the transformer, and the break may be accomplished manuallyor by automatic controls.

We claim as our invention:

1. The method of melting a charge comprising applying heat to the chargeat a substantially constant rate by means of an electric current ofsubstantially constant value, and intermittently interrupting thecurrent to produce predetermined intervals of non-heat application bysaid current, said intervals being chosen to permit the temperature ofthe charge to increase progressively and being dependent upon the valueof the current and the weight of the charge.

2. The method of melting a charge containing elements having differentvaporizing and melting points, comprising applying heat to the charge bymeans of current 01' a substantially constant value; intermittentlydiscontinuing said current, the temperature of the charge progressivelyincreasing until it reaches a temperature at which the impurities aresubstantially all vaporized, but which is below the vaporizing point ofany ele- 'ments to be retained, and then applying said currentcontinuously until all of the remaining elements of the charge aremelted, the intermittent intervals being predetermined dependent uponthe value of current used and weight of charge to be melted.

3. The method of forming a tungsten carbide alloy comprising placing acharge of tungsten and other substances in a carbon crucible, applyingheat to the crucible by means of an electric current of substantiallyconstant value, and intermittently discontinuing said current atintervals permitting the temperature of the charge to increaseprogressively, the intermittent intervals being predetermined dependentupon the value of current used and weight of charge to be melted.

4. The method of forming a tungsten carbide alloy comprising placing a.charge of tungsten and other substances having different vaporizing andmelting points in a carbon crucible, applying heat to said crucible bymeans of an electric current of a substantially constant value,intermittently discontinuing said current, until the charge reaches atemperature at which the impurities are substantially all vaporized, butwhich is below the vaporizing point of any elements to be retained, andthen applying said current continuously until the remaining elements aremelted and the tungsten has been alloyed with sufficient carbon absorbedfrom the crucible, the intermittent intervals being predetermineddependent upon the value of current used and weight of charge to bemelted.

5. The method of melting high melting point substances together withvaporizable constituents which comprises charging the same in a crucibleand passing a current of substantially constant value through the chargeintermittently at first to permit venting of gases and continuouslythereafter in order to obtain an initial slow progressive heatingincrease followed by a final desired high temperature heating to meltthe charge; the intermittent intervals being predetermined dependentupon the value ofcurrent used and weight of charge to be melted.

JOHN A. ZUBLIN. JOHN S. GOODWIN.

